I'm generally against the death penalty (I say "generally" just to avoid talking in absolutes), but... In this case, and other's like Timothy McVeigh, I can sort of appreciate the decision. When Tsarnaev is killed this situation is over. While, obviously, there are no appeals for sentences like his, that he continues to exist gives more possibility of him being relevant 10, 15, 20, or 50 years from now. With him dead, that possibility of future stories about him or some person making an appeal about him, or something, are over.
I've been at the Boston Marathon every year for the last 10, never at the finish line, and obviously I wasn't directly affected by any of this other than a egocentric "damn I live 20 miles from this nightmare" sort of thing... But even while I'm against the death penalty and I get no joy in any "sick revenge fantasy" as other people have put it, for me, when he's dead, it's the end of Tsarnaev without some glimmer of a chance that he'll ever be relevant in my life, without some teenage girl on Twitter posting "Free Dzokar" because he's supposedly handsome, or some right wing conspiracy theorist thinking that this was a CIA false flag government takeover program therefore he's innocent.
As uncomfortable as the death penalty is for me, for some illogical reason, the elimination of relevance of Tsarnaev is more comforting to me than the slim chance of his relevance 10, 20, or 50 years from now.
It's one of those times when my feelings lack a convincing explanation.